1. Field of the Invention
The present invention relates to a piezoelectric resonator, and more particularly, to a piezoelectric element for use as an oscillator in a communication filter and a clock generator.
2. Description of the Related Art
As one of conventional piezoelectric elements, known is a piezoelectric element produced by polarizing a piezoelectric ceramic body having a layered perovskite structure perpendicular to the orientation axis of the piezoelectric ceramic body. In such a piezoelectric element, electrodes are formed on both of the end faces of the piezoelectric ceramic body. Piezoelectric vibration can be excited by applying an electric field in the polarization direction.
Moreover, the inventors of the present invention have proposed a piezoelectric element of such a type that interdigital electrodes are formed on the front and back main faces of a single plate piezoelectric ceramic body for excitation. This piezoelectric element does not use a lamination process with a large number of steps. Accordingly, the piezoelectric element can be utilized conveniently, industrially, and has a high reliability with no problem on the optimization of coupling of an electrode interface to a ceramic.
Referring to the piezoelectric element produced by forming electrodes on both of the end faces of a piezoelectric ceramic body having a layered perovskite structure, and polarizing the ceramic in the direction perpendicular to the orientation axis, it is necessary for the piezoelectric element to have a sheet or rod shapes structure elongating in the electric field application direction. Such a structure has the problem that the electrostatic capacitance is low and the impedance is high, so that it is difficult to achieve impedance matching to a circuit.
Moreover, as regards the piezoelectric element in which the interdigital electrode is formed on a main face of a single plate piezoelectric ceramic body, e.g., the back main face thereof, it is necessary to reduce the thickness of the element in order to polarize the whole of the element when the piezoelectric element is used at a high frequency. This causes the problem that the mechanical reliability of the piezoelectric element can not be enhanced when the element is used at a high frequency.
Accordingly, it is a main object of the present invention to provide a piezoelectric element which has a low impedance, a high electromechanical coefficient and a high-frequency low-loss characteristic, even though the element uses a piezoelectric ceramic body having a layered perovskite structure.
Moreover, it is another object of the present invention to provide a method of producing the above piezoelectric element.
To achieve the above objects, there is provided a piezoelectric element according to the present invention which comprises a piezoelectric ceramic body having a layered perovskite structure of which at least the C axis of the crystallographic axes is oriented, and which is polarized substantially in the perpendicular to the orientation direction of the C axis, and plural electrodes arranged on respective planes substantially in parallel to the polarization direction of the piezoelectric ceramic body in the piezoelectric ceramic body, the plural electrodes being arranged so that the electrodes connected to one potential and the electrodes connected to the other potential contain parts thereof which are adjacent to each other.
Preferably, the plural electrodes are arranged in an interdigital electrode form.
Plural electrodes may be provided on multistages in the orientation direction of the C axis. In this case, electrodes overlapped with each other are connected to the same potential.
Preferably, the piezoelectric ceramic body is polarized in two opposite directions between electrodes overlapped with each other and other electrodes overlapped with each other.
Moreover, there is provided a method according to the present invention of producing a piezoelectric element which comprises the steps of: using a piezoelectric body material having a layered perovskite structure to prepare a green sheet; printing an electrode paste so that the plural electrode pastes printed are arranged substantially parallel to each other on the green sheet; laminating the green sheets so that the printed electrode pastes are interposed between the piezoelectric body materials, respectively, whereby a laminate is formed; and firing the laminae, and thereafter, polarizing the fired laminate, the piezoelectric body materials being polarized substantially perpendicular to the orientation direction of the C axes after the C axes of the piezoelectric body materials are oriented substantially in parallel to the lamination direction of the laminate.
In the piezoelectric element of the present invention, the piezoelectric ceramic body having a layered perovskite structure of which at least the C axis of the crystallographic axes is selected and oriented is used, and is polarized substantially in the direction perpendicular to the orientation axis, and the plural electrodes are arranged on planes substantially parallel to the polarization directions. Thereby, an electric field can be applied substantially perpendicular to the C axis of the piezoelectric ceramic body. Moreover, the polarization direction of the piezoelectric ceramic body has a component thereof substantially perpendicular to the C axis selected and oriented. Therefore, a single mode piezoelectric vibration can be realized.
Preferably, the plural electrodes are arranged in an interdigital electrode form so that an electric field is applied in the above direction.
In the piezoelectric element, the electrodes may be provided on multistages in the orientation direction of the C axis. At this time, electrodes overlapped with each other are connected to the same potential. Thus, an electric field can be applied in the same direction in the areas where the polarization direction is the same.
Moreover, preferably, the piezoelectric ceramic body is polarized in two opposite directions between electrodes overlapped with each other and other electrodes overlapped with each other. Thereby, areas are formed in which the ceramic is polarized in the opposite directions to the perpendicular to the C axis selected and oriented. An electric field is applied in the same direction as the polarization direction, so that a single mode piezoelectric vibration can be obtained.